The present invention relates generally to integrated circuits and more particularly to a circuit and/or method for detecting fuse states in an integrated circuit.
Fuse elements (such as laser fuses, anti-fuses, transistors, etc.) are used to establish a variety of settings within an integrated circuit. In a memory device, for example, fuse elements may be used to establish the part configuration (e.g., X4/X8/X16) and/or various trimming settings (such as voltage levels, part timings, row and column repair/replacement, etc). One type of fuse that is utilized is referred to as a “laser fuse.” Typically, the memory device includes one or more strips of metal within a circuit that may be programmed, or “blown”, using a laser. The blown laser fuse causes a latch to be set to a specific state within the circuit. The state of the latch may then be used to set the operation of the memory device. Because a laser is used to program the laser fuses, the fuse setting process must be completed prior to memory device packaging. Accordingly, laser fuse settings cannot be modified after packaging is complete.
Another type of fuse element that is utilized is referred to as an “anti-fuse”. In an “unblown” (i.e., unprogrammed) condition, an anti-fuse functions as a capacitor presenting a very high resistance on the order of 10 Megohms to the circuit in which it is placed. In a “blown” (i.e., programmed) condition, the connections of the anti-fuse are shorted together (using a very high voltage, for example) providing a relatively low resistance (approximately 200 to 500 ohms) path through the anti-fuse. Because a laser is not required for programming, anti-fuses can be programmed after device packaging is completed.
FIG. 5 is a detailed view of a fuse state detection circuit 100 according to the prior art. The fuse state detection circuit 100 produces an output signal (X) having either a logic 1 (i.e., Vcc level) or a logic 0 (i.e., ground level). The output signal X is responsive to the state of the anti-fuse 106. For example, the fuse state detection circuit 100 illustrated in FIG. 5 produces an output signal X with logic 1 (i.e., Vcc) if anti-fuse 106 is “blown” and with logic 0 (i.e., GND) if anti-fuse 106 is “unblown.”
The operation of the prior art fuse state detection circuit 100 will now be discussed in more detail. The signals CGND, FA, BSELY_, MRG_, and DVC2F are used for programming the anti-fuse 106 and/or reading the state of anti-fuse 106. When the memory device is operating under normal conditions, it is assumed that CGND=logic 0, FA=logic 0, BSELY_=logic 0, MRG_=logic 0, and DVC2F=logic 1. Under these conditions, the fuse state detection circuit 100 can be “simplified” as illustrated in FIG. 6.
The simplified circuit illustrated in FIG. 6 is basically a simple latch with a reset p-channel transistor M2 and anti-fuse 106. The output of the latching circuit (i.e., signal X) is responsive to (and thus indicates) the state of anti-fuse 106. As seen in FIG. 6 when anti-fuse 106 is “blown”, anti-fuse 106 functions as a resistor and the voltage at node A is set at logic 0 (i.e., pulled to GND). Inverter 104 inverts the voltage present at node A such that when anti-fuse 106 is “blown,” the output signal X of fuse state detection circuit 100 is set to logic 1 (i.e., representing that anti-fuse 106 is blown).
When anti-fuse 106 is unblown, node A is floating because anti-fuse 106 acts as a capacitor, isolating node A from GND. In this case, the state of the Read Fuse signal (RDFUS_) is used to determine the logic state of output signal X. Initially, RDFUS_pulses “low,” turning on p-channel transistor M2 and pulling node A to logic 1 (i.e., to Vcc). Accordingly inverter 104 inverts the voltage present at node A setting the logic state of output signal X to logic 0 (i.e., representing that anti-fuse 106 is unblown). When RDFUS_returns “high” (i.e., when the “low” pulse times out), p-channel transistor M2 is turned off and feedback inverter 102 latches the logic state of output signal X at logic 0 (i.e., which continues to represent that anti-fuse 106 is unblown).
As is apparent from the above-description, when anti-fuse 106 is unblown the output state of fuse state detection circuit 100 is dependent upon the ability of the feedback inverter 102 to latch. Due to its weak driving signal, however, the latching ability of feedback inverter 102 may be easily overcome. Thus, the fuse state detection circuit 100 is susceptible to negative triggering events. A negative trigger event refers to any event that causes a node or point within a circuit to undesirably change states (e.g., to cause a node or point to change from a negative logic to a positive logic and/or to cause a node or point to change from a positive logic to negative logic). A negative triggering event may or may not cause the output of the fuse state detection circuit 100 to produce and erroneous signal X. Negative triggering events may be caused by voltage spikes due to signal coupling, a radiation particle strike, power bus drop, and/or a collection of positive or negative charge which can lead to circuit instability, among others.
If a negative triggering event occurs, the fuse state detection circuit 100 may produce an erroneous output signal X. For example, assume that anti-fuse 106 is unblown. As discussed above, node A is set to logic 1 and the output signal X set at logic 0. If a negative triggering event overcomes the latching ability of feedback inverter 102, node A is forced to logic 0 and output signal X is forced to logic 1 erroneously representing that anti-fuse 106 is blown. If this were to happen, the device responsive to output signal X may not work properly. Fuse state detection circuit 100 requires that a new RDFUS_pulse be applied to reset the latch.
Although periodically pulsing RDFUS_to reset the latch 102 seems like an easy solution to the problem, such pulsing typically requires large amounts of current. More specifically, because some devices may include up to 10,000 anti-fuses, each with a fuse state detection circuit, a large amount of current is consumed each time the RDFUS_signal is pulsed. Accordingly, during normal operation RDFUS_is pulsed very infrequently to reduce the amount of current consumed by the device.
Thus, there exists a need for a fuse state detection circuit that resists negative triggering events, that resets itself without using a RDFUS_pulse should a negative triggering event cause the fuse state detection circuit to produce an erroneous output signal, and which overcomes other limitations inherent in prior art.